Conventionally known decorative sheets useful for surface decoration include the following:
(1) A precoated sheet prepared by providing an embossed pattern on one surface of a substrate sheet by means of an ordinary printing method, using an ink which has a high solid, such as a pigment, content, and then forming thereon a surface layer by using a varnish composed of an aminoalkyd resin, a urethane resin or the like (Japanese Patent Laid-Open Publications Nos. 76115/1977 and 71908/1978).
(2) A decorative sheet prepared by prepared by providing a pattern on one surface of a substrate sheet by the use of an ink which contains an ordinary printing ink and a foaming agent, and then expanding the foaming agent to partly provide a built-up portion in the pattern (Japanese Patent Publication No. 12285/1961).
(3) A decorative sheet prepared by laminating a transparent sheet to the printed surface of a pattern-printed colored substrate sheet, and, when necessary, forming an embossed pattern during this laminating process (Japanese Patent Laid-Open Publication No. 14312/1983).
(4) A resin-impregnated decorative plate prepared by printing a pattern on a substrate sheet which is excellent in resin-impregnating, ability, impregnating the substrate sheet with a resin, and then curing the resin (Japanese Patent Publication No. 4540/1951).
The above-described conventional decorative sheets are not necessarily satisfactory when they have a natural pattern such as a grain pattern. There is a limit to the formation of a natural pattern such as a grain pattern by an artificial printing technique. It is the present situation that only such a decorative sheet that has an artificial and unnatural appearance, lacking in natural feeling is obtained by any of the conventional methods.
In addition, the above conventional decorative sheets (1) to (4) have the following problems to be solved.
(1) To prepare the conventional precoated sheets, an embossed pattern is provided by printing. However, the ink used is poor in printability, so that the pattern is limited by the color tone and the density, and thus the desired embossed pattern cannot be fully obtained. Further, a varnish composed of an aminoalkyd resin, a urethane resin or the like, by which a resinous surface layer is formed, is hard and brittle. Therefore, only a small amount of the varnish can be applied, and the resulting layer is thus poor in abrasion resistance.
(2) The decorative sheets with portions built-up by the expansion of a foaming agent may have a good three-dimensional pattern. However, many of the decorative sheets of this type are composed of polyvinyl chloride, which is a thermoplastic resin, so that there has been such a problem that they are poor in both abrasion resistance and heat resistance. Further, the pattern itself is formed by bulging in these decorative sheets; this is unsuitable to obtain a decorative sheet which has a pattern resemble to a natural pattern such as a grain pattern.
Furthermore, in order to provide built-up printed portions having heat resistance, an ink has been prepared by the use of a binder which contains as main components a reactive acrylic resin and a vinyl chloride copolymer. However, it is difficult to add a pigment to this ink in such an amount as is adopted to an ordinary ink. For this reason, the range of the type and the amount of a pigment suitable to obtain a desired density is narrow, and only an unsatisfactory color tone is obtained. For example, when a pigment such as iron oxide or titanium oxide is incorporated, the ink loses its fluidity, and becomes dilatant even when its viscosity is controlled by a solvent. Such an ink is unsuitable for printing. Moreover, it has been difficult to prepare a printing ink capable of providing a pattern with a high color density.
(3) The conventional decorative sheets prepared by laminating a transparent sheet to the printed surface of a pattern-printed colored substrate sheet are excellent in abrasion resistance because the pattern layer is protected by the transparent sheet. However, many of the decorative sheets of this type are composed of polyvinyl chloride, which is a thermoplastic resin, so that there is such a problem that they are poor in heat resistance.
(4) The conventional decorative plates impregnated with a thermosetting resin have moderate heat resistance and abrasion resistance. However, it is difficult to form an embossed pattern which is fully corresponding with the desired pattern. In addition, the decorative plates of this type are lacking in flexibility, and also expensive.
The present invention has been accomplished in the light of the above-described prior art. An object of the present invention is therefore to provide a decorative sheet which has an appearance, feeling and touch which are remarkably resemble to those of the grain of natural wood, and which is also excellent in material characteristics such as abrasion resistance and heat resistance.